Electrophoresis gels are useful for the separation of biological molecules including proteins, RNA, DNA and carbohydrates. The transfer of electrophoretically separated material by blotting has become a standard procedure for sensitive and specific analysis of electrophoresis gels.
Early blotting techniques involved the use of capillary action to transfer material from electrophoresis gels. These techniques were slow and inefficient requiring up to 16 hours or more to transfer all of the material from a gel.
Blotting under the influence of an electric field offers significant advantages over capillary transfer techniques. In particular, such blotting is generally quicker and more efficient than capillary transfer. Capillary transfer and blotting using an electric field both require that a gel be placed in contact with an immobilizing-membrane to which the electrophoretically separated biological molecules are transferred. This combination is sometimes referred to as a "transfer stack." The stack usually also includes one or more sheets of buffer-wetted filter paper sheets on both the top and bottom of the stack for ease of handling and to protect fragile membranes from contamination or damage. In capillary transfer the transfer force is the absorptive potential of the immobilizing transfer membrane and, usually, additional dry (bibulous or highly absorptive) paper on the opposing side of the membrane. In blotting using an electric field, the driving force is the electrode potential between the two electrodes placed on either side of the transfer stack.
Although more efficient than capillary blotting, electric blotting techniques can also suffer from significant disadvantages. These disadvantages include: non-uniform electric fields at the transfer stack surfaces and voltage leakage around the surfaces of the electrodes both which result in inefficient or inconsistent transfer of DNA.
One prior art solution (U.S. Pat. No. 4,589,965) involves sandwiching a gel/immobilizing transfer material between two electrodes of equal size. Although this method was more efficient than previous techniques, it still suffered from inconsistent transfer problems. Gel/transfer material "sandwich" surfaces (i.e. blotting membrane surfaces) are inherently uneven. As a result, it is difficult to obtain consistent contact between the electrode surface and the blotting membrane. Accurate transfer across the entire gel is critical for subsequent analyses of the blotted macromolecules.
Another disadvantage of prior blotting apparatus is the cost of the relatively large surface area electrodes, in part due to the use of platinum on the electrodes. One solution to this problem is found in the GeneSweep.TM. brand blotting apparatus sold by Hoefer Scientific Instruments of San Francisco, Calif., assignee of the present application. In the GeneSweep.TM. device, one of the electrodes is made smaller than the opposing electrode, but movable along the upper surface of the blotting membrane. However, this device still suffers from a lack of consistent contact between the electrode surface and the blotting membrane.
It would therefore be advantageous to have a device which would transfer electrophoretically separated material rapidly, reproducibly and uniformly, yet avoid the drawbacks of prior systems.